In metal-extrusion processes of the type wherein a billet of a material such as aluminum or copper is extruded by a press ram through a die and a piercing mandrel is displaceable within that ram to form a passage in the extruded product in the production of extruded metal tubing, it is common for the hollow press ram to be carried by a traveling beam which serves to receive and guide the piercing mandrel which passes through the press ram. That piercing mandrel is normally carried by a traverse which itself is guided in the traveling beam and which, in turn, is connected to a piercing piston which can be guided or disposed in part in the main press piston coupled with the traveling beam.
In the production of tubing from, for example, extrudable metals like aluminum and copper by extrusion press methods, both direct and indirect extrusion presses are used. The direct press is, however, the predominant apparatus for this purpose. An important characteristic of the direct extrusion press is that the die and the receiver are fixed with respect to one another. By contrast, in an indirect extrusion press, the die penetrates into the receiver. The die then has a hollow ram fitted at its head with a die holder carrying the die. On one side the receiver is sealed by a closure member.
The billet and tube press of this type and the pressing programs for operating same with a variety of materials are described for example in "Sonderdruck aus Zeitschrift fur Metallkunde 51 (1960) 2, pages 3 to 10".
In the operation of such direct or indirect billet and tubing presses it has been found that the traveling beam during production, because of the axial pressing forces, is stressed in compression. This gives rise to a spreading or bulging at approximately the center of the two opposite side walls of the frame-like traveling beam. Since the piercing mandrel traverse is guided between these side walls, it is almost impossible in practice to prevent a shift in the position of the traverse off center. With such shifting of the guide surfaces for the traverse, the passage formed in the product tends to be eccentric and narrow fabrication tolerances of the tubing produced cannot be effectively maintained.